1. Technical Field
The present invention relates to an optical fiber amplifier and, more particularly, to a multichannel 3-stage optical fiber amplifier.
2. Related Art
The greatest merit of an optical transmission network is the capability of transmitting much data information over a greater distance. However, when a signal light is transmitted hundreds of kilometers or farther, a loss of 10 dB or more occurs in multiple of 10 km, even when using an optical fiber having a loss of 0.2 dB/Km per length unit. Thus, amplifiers which compensate for this loss must be used for light transmission.
An electronic amplifier using an optical-to-electrical, electrical-to-optical converter prevents signal noise and reproduces a waveform. However, the electronic amplifier limits the speed of an optical transmission network due to delay time as well as low processing speed required for signal restoration. Therefore, a technique for amplifying a signal light directly as light is required. An erbium doped fiber amplifier (EDFA), obtained by adding a rare-earth metal such as erbium (Er) to a general optical fiber, is widely used as the above-described fiber-direct amplifier because of its simple structure and relatively excellent performance.
However, when a wavelength division multiplexing (WDM) method, in which signals are carried on different wavelengths and signals of several channels are transmitted simultaneously, is used to transmit large amount of information via a single optical fiber, certain characteristics--particularly, gain and noise--must have a constant value at every wavelength of each channel in the EDFA. That is, in the WDM EDFA, the power of an input signal increases in proportion to the number of channels, as compared to the other single channel EDFAs. For example, an EDFA (in-line EDFA) used as a single channel repeater usually receives an input signal of about -20 dBm, but an 8-channel WDM EDFA receives -20 dBm for each channel, providing a total input power of -11 dBm. However, the entire saturation output power of the EDFA is constant so that output power for each channel is reduced, and the entire gain or the gain for each channel is accordingly reduced.
The EDFA amplifies the input signal, and simultaneously adds spontaneous emission noise to signals of other wavelengths. When a signal passes through several EDFAs during long distance transmission, the spontaneous emission noise is amplified and accumulated with the signal. The thus-accumulated noise degrades the signal-to-noise ratio, and is input to an amplifier with the signal. Thus, population inversion within an optical amplification fiber is reduced, thereby reducing the amplification factor of the original signal.
It is impossible to presume how many channels, among the input channels of a WDM EDFA, the input signal enters. The number of channels input to the amplifier can vary with the number of channels added or dropped. The entire power of the input signal varies depending on the number of input channels, so that the total gain of the EDFA can also be changed.
There are several methods of equalizing gain and noise according to wavelength in an EDFA: (1) changing the composition of an optical amplification fiber by adding aluminum (Al), ytterbium (Yb) and samarium (Sm) to the optical fiber, or changing the characteristics of the optical amplification fiber using a fluorite optical fiber instead of an optical fiber based on silica; (2) attaching, to an existing EDFA ,a Mach-Zehnder filter, an acoustic filter, a Fabric/Perot filter and a filter such as a fiber grating; (3) connecting EDFAs having different characteristics to each other.
However, these methods increase the size and cost of the EDFA, and cannot remove the aforementioned emission noise.